Semiconductors having equal numbers of acceptor and donor impurities



Dec. 31, 1963 F. RN ETAL 3,116,260 SEMICONDUCTORS HAVING EQUAL NUMBERS OF ACCEPTOR 1 AND DONOR IMPURITIES Filed Jan. 29, 1960 (%)TRANSM|SSION 8 3 8 8 8 9 o l l l I l l 9 Q 5, Q o a A 0.2 2 "P 0 Q 0 '2 Q g 1-2 2% u 3. '6 c c 6 0 NJ a l l I l l INVENTORS. FRANK STERN BY JACK R. DIXON ROBERT M. TALLEY GUI/24,01,

Unite States Hill "ENG EQUAL NUhillhll DQNQR llyilURl'llES Silver Spring, Md, and Calii, assignors to the SEli HCGNBUQTURS F ACCEPTGR ANB Frank Stern and Each R. Dixon,

Robert M. "if alley, Santa Barbara, the United States of America as represented by Secretary oi the l avy Filed Jan. 2?, 156b, Ser. No. 5,572

ms. (Ql. 252-581) {Granted under Title 35, Qode {1952), see. 25s

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to semiconductors and more particularly to semiconductor photodetectors or filters in the infrared region of the light spectrum.

According to present day atomic theory, the electrons of an atom are considered to orbit at certain characteristic distances around the nucleus. ll of the electrons in a particular orbit or shell are considered to be at the same energy level. In a solid, the discrete energy levels are re placed by energy bands which result from interaction between the plurality of atoms. As a first approximation, a particular energy level in an atom is associ ted with a particular energy band in the solid. The conduction properties of a solid depend on the number of electrons in the various bands. The highest energy band which is predominately filled with electrons is referred to as the valence band. The conduction band is the energy band immediately above the valence band. The difference in energy between the top of the valence band and the bottom of the conduction band is called the forbidden energy gap since electrons cannot exist the ein. In order to transfor an electron from one band to another, sufficient energy must be imparted thereto to move it completely across the energy gap. Ii insuificient energy is imparted to such an electron, it cannot move from the band where it was ori inally located. In the case of an insulator there are practically not electrons in the conduction band while a semiconductor has a considerably greater number of electrons in the conduction band.

Light which has more energy than the width of an energy gap can induce electronic transitions from the corn duction to the valence band. Such light is therefore strongly absorbed by the material upon which it impinges. Light which has less energy than the energy gap can not produce such transitions and will consequently be generally trans--itted through the material. Thus, sufficiently long wavelength light is transmitted while shorter wavelength light absorbed by a pa 'cular 0nd "tor. An abrupt change in cerisnc or a solid occurs at wav h correspondir to the width of the energy gap. T cut-off waveleng h or transmission edge. is erseiy proportional to e less than cut-off u avelcngth ll will be absorbed ereby, but lenth greater than the c -05? Wav transn ied through no material. As a result, tion of the transmiss n e ge can be altered by va energy gap of a particular semicondu tor.

Semiconductors are now known that have energy gaps between the valence and conduction bands ranging from 0.1 electron volt to several electron volts. Previousl it Was possible to decrease the energy gap by adding large concentrations of donor impurities to the semiconductor. The material thus obtained was not suitable as a photofilter because of the optical obsorption associated with the added carriers. Another way to change the energy gap is by alloying one semiconductor with another. Also, attempts have been made to decrease the energy gap by ice greatly changing the temperature and/ or pressure of the emiconductor after it has been made. The preceding processes are not easy to perform and they do not produce results which are practical in many cases.

In the present invention, the energy gap between the valence and conduction bands of a semiconductor is re duced by distributing equal numbers of acceptor and donor impurity atoms throughout that semiconductor. Impurity atoms having an excess number of electrons, i.e., donor atoms, are inserted in great enough quantities into the semiconductor to lower the bottom of the conduction band. Enough impurity acceptor atoms are added so that excess donor electrons Will be removed from the semiconductor conduction band. This serves to decrease the optical absorption due to the conduction electrons, or to increase the long wavelength optical transmission of the material. By controlling the amount of impurities thus inserted into the semiconductor, the energy gap thereof can easily be controlled.

Accordingly, it is an object of this invention to provide a narrower energy gap than normal for a particular semiconductor.

It is another object to provide a new and improved semiconductor having an energy gap that is decreased.

A further object is to provide a photodetector having a controlled cut-oil wavelength.

An additional object is to provide an optical filter having high infrared transmission with a variable cut-oil wavelength.

A still further object is to provide a new and improved semiconductor having an infrared obsorption character istic that can be controlled. 7

Other objects and many of the attendant advantages of this invention will be appreciated as the same becomes better understood by reference to the following etailed description when considered in connection with the accompanying drawing.

The FlGURE is a graph comparing the optical transmission properties of a heavily doped compensated indium arsenide sample v ith relatively pure sample of the sol size and type material. Curve ill shows the percentage of optical transmission versus variations in wavelength of infrared light impinging on a relatively pure sample having 4x19 conduo. n electrons per cubic centimeter and approximately 4 i0 donor atoms per cubic centimeter. Curve 12 is a similar representation for an indium arsenide sample which has been compensated by adding 5X10 donor atoms oi sulphur per cubic cent cter of the semiconductor as well as an approximately equal number of Zinc acceptor atoms. The impuri 'es which have been added almost completely compensate each other. 'lfhis results in a semiconductor having 9 X16 conduction electrons per cubic centimeter, which is only 0.69% of the total impurity concentration.

it is see from the figure that the cut-oil wavelength of the compensated semiconductor has been increased over that of the purer sample. The pure material has a cut-off wavelength of approximately 3.75 microns while the doped specimen has a cut-oil Wavelength in the neighborhood of 4.75 microns. As the amount of impurities is increased, the cut-oil wavelength will likewise be increased. The expected upper limit is around 19 impurities per cubic centimeter. The figure also illustrates that the compen sated sample has poorer long wavelength properties than the pure sample. Nevertheless, this long wavelength transmission is large enough to be useful. It is thus seen that the absorption or transmission edge can be greatly varied by the addition of sufficiently large and equal numbers of donor and acceptor atoms.

The change in infrared transmission properties occurs because the energy gap beteen the conduction and valence and bands of a semiconductor decreases when the donor acceptor atoms are distributed therethrough. The donor impurity concentrations must be large enough to lower the bottom of the semiconductor conduction bands. It is also necessary that the acceptor impurity concentration be large enough to remove excess electrons from the semiconductor conduction band.

By the addition of 5x10 atoms of sulphur and approximately a light number of zinc atoms per cubic centimeter of pure indium arsenide, the energy gap thereof Will be reduced by approximately 0.67 electron volt. The pure sample has an energy gap of approximately 0.33 electron volt while the compensated sample has an energy gap of approximately 0.26 electron volt. Light having energy greater than 0.33 electron volt, i.e., Wavelengths less than 3.75 microns, Will be absorbed by the pure sample but light having energy greater than 0.25 electron volt, i.e., Wavelength less than 4.75 microns, will be absorbed by the doped sample. Absorption takes place because the light energy is suthcient to move electrons from the conduction to the valence band of the material upon which it is impinging. An increase in the wavelength of light causes a decrease in the energy thereof. When the wavelength is sufiiciently increased there is not enough energy imparted by the light on the semiconductor to cause electrons to cross the energy gap. This results in transmission of the light through the semiconductor.

It has been found that the minimum concentration of an impurity required to move the band edge is approximately M a 3 X 102s i mic atoms per cubic centimeter of the semiconductor. In this expression M is the effective mass of the carriers, in a band, i.e., valence or conduction band eiiective mass, depending on which is smaller; in is the mass of a free electron, i.e., 9.1x 19- grams; and k is the semiconductor dielectric constant. Since M is generally greater for the valence hand than for the conduction band of a particular material, the minimum concentrantion usually applies to the number of donor atoms.

In some semiconductors the role of the acceptor and donor atoms will be reversed if the valence band efiective mass is smaller than the conduction band effective mass. In such a case the number of acceptor atoms distributed through the semiconductor must be sufiicient to remove electrons from the valence band and thereby raise the top of that band. The number of donor atoms must be great enough to remove the excess holes thus formed in the semiconductor valence band. Therefore, in any semiconductor the number of one type of impurity inserted must be sufficient to shift the edge of the energy band with which it corresponds and the concentration of the other impurity must be sufficient to compensate the excess carrier concentration thus formed.

The impurities may be distributed evenly throughout the semiconductor by adding them to the original melt when the semiconductor is being made. Also, the impurities may be added by diffusion through the semiconductor. This may be done by any well known method such as zone leveling as disclosed on pages 59 and 60 of volume 6, part A, entitled Solid State Physics, of the series Methods of Experimental Physics, edited by K. Lark-Horovitz and Vivian A. Johnson, and published by Academic Press of New York and London in 1959.

It is to be understood that the invention is not limited to any particular semiconductor or impurities. For example, decreases in the energy gaps of indium antimonide and indium phosphide have been observed when large and substantially equal amounts of donor and acceptor impurities have been inserted therein.

A unique emiconductor detector or filter having a controlled infrared absorption charactcristic wherein the energy gap may be varied has been herein disclosed.

Obviously many modifications and variations of the present invention are possible in the light or" the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is new and desired to be secured by Letters Patent of the United States is:

1. A material responsive to infrared light comprising an indium arsenide semiconductor, sulphur donor impurity atoms evenly distributed throughout said semiconductor, zinc acceptor impurity atoms evenly distributed throughout said semiconductor, the number of said sulphur atoms being approximately equal to the number of said Zinc atoms, the number of said sulphur atoms being above the minimum required to lower the bottom of the indium arsenide conduction band, whereby the cutoff wavelength of said semiconductor is selectively increased by the number of said impurity atoms.

2. The material of claim 1 wherein approximately 5x10 atoms of sulphur are contained in each cubic centimeter of indium arsenide.

3. The material of claim 1 wherein the number of said Zinc atoms is sufiicient to remove excess donor electrons from the indium arsenide conduction band.

References Cited in the file of this patent UNITED STATES PATENTS 2,759,861 Collins et al Aug. 21, 1956 2,765,385 Thomsen Oct. 2, 1956 FOREIGN PATENTS 815,674 Great Britain July 1, 1959 

1. A MATERIAL RESPONSIVE TO INFRARED LIGHT COMPRISING AN INDIUM ARSENIDE SEMICONDUCTOR, SULPHUR DONOR IMPURITY ATOMS EVENLY DISTRIBUTED THROUGHOUT SAID SEMICONDUCTOR, ZINC ACCEPTOR IMPURITY ATOMS EVENLY DISTRIBUTED THROUGHOUT SAID SEMICONDUCTOR, THE NUMBER OF SAID SULPHUR ATOMS BEING APPROXIMATELY EQUAL TO THE NUMBER OF SAID ZINE ATOMS, THE NUMBER OF SAID SULPHUR ATOMS BEING ABOVE THE MINIMUM REQUIRED TO LOWER THE BOTTOM OF THE INDIUM ARSENIDE CONDUCTION BAND, WHEREBY THE CUTOFF WAVELENGTH OF SAID SEMICONDUCTOR IS SELECTIVELY INCREASED BY THE NUMBER OF SAID IMPURITY ATOMS. 